Generation of Bradyrhizobium japonicum mutants with increased N2O reductase activity by selection after introduction of a mutated dnaQ gene

Natural variants of molybdate transporters contribute to yield traits of soybean by affecting auxin synthesis

Soybean (Glycine max) is a crop with high demand for molybdenum (Mo) and typically requires Mo fertilization to achieve maximum yield potential. However, the genetic basis underlying the natural variation of Mo concentration in soybean and its impact on soybean agronomic performance is still poorly understood. Here, we performed a genome-wide association study (GWAS) to identify GmMOT1.1 and GmMOT1.2 that drive the natural variation of soybean Mo concentration and confer agronomic traits by affecting auxin synthesis. The soybean population exhibits five haplotypes of the two genes, with the haplotype 5 demonstrating the highest expression of GmMOT1.1 and GmMOT1.2, as well as the highest transport activities of their proteins. Further studies showed that GmMOT1.1 and GmMOT1.2 improve soybean yield, especially when cultivated in acidic or slightly acidic soil. Surprisingly, these two genes contribute to soybean growth by enhancing the activity of indole-3-acetaldehyde (IAAld) aldehyde oxidase (AO), leading to increased indole-3-acetic acid (IAA) synthesis, rather than being involved in symbiotic nitrogen fixation or nitrogen assimilation. Furthermore, the geographical distribution of five haplotypes in China and their correlation with soil pH suggest the potential significance of GmMOT1.1 and GmMOT1.2 in soybean breeding strategies.

Bradyrhizobium ottawaense efficiently reduces nitrous oxide through high nosZ gene expression

N2O is an important greenhouse gas influencing global warming, and agricultural land is the predominant (anthropogenic) source of N2O emissions. Here, we report the high N2O-reducing activity of Bradyrhizobium ottawaense, suggesting the potential for efficiently mitigating N2O emission from agricultural lands. Among the 15 B. ottawaense isolates examined, the N2O-reducing activities of most (13) strains were approximately five-fold higher than that of Bradyrhizobium diazoefficiens USDA110T under anaerobic conditions. This robust N2O-reducing activity of B. ottawaense was confirmed by N2O reductase (NosZ) protein levels and by mitigation of N2O emitted by nodule decomposition in laboratory system. While the NosZ of B. ottawaense and B. diazoefficiens showed high homology, nosZ gene expression in B. ottawaense was over 150-fold higher than that in B. diazoefficiens USDA110T, suggesting the high N2O-reducing activity of B. ottawaense is achieved by high nos expression. Furthermore, we examined the nos operon transcription start sites and found that, unlike B. diazoefficiens, B. ottawaense has two transcription start sites under N2O-respiring conditions, which may contribute to the high nosZ expression. Our study indicates the potential of B. ottawaense for effective N2O reduction and unique regulation of nos gene expression towards the high performance of N2O mitigation in the soil.

Coupling of the engineered DNA "mutator" to a biosensor as a new paradigm for activation of silent biosynthetic gene clusters in Streptomyces

DNA replication fidelity in Streptomyces bacteria, prolific producers of many medically important secondary metabolites, is understudied, while in Escherichia coli it is controlled by DnaQ, the ϵ subunit of DNA polymerase III (DNA PolIII). Manipulation of dnaQ paralogues in Streptomyces lividans TK24, did not lead to increased spontaneous mutagenesis in this bacterium suggesting that S. lividans DNA PolIII uses an alternative exonuclease activity for proofreading. In Mycobacterium tuberculosis, such activity is attributed to the DnaE protein representing α subunit of DNA PolIII. Eight DnaE mutants designed based on the literature data were overexpressed in S. lividans, and recombinant strains overexpressing two of these mutants displayed markedly increased frequency of spontaneous mutagenesis (up to 1000-fold higher compared to the control). One of these ‘mutators’ was combined in S. lividans with a biosensor specific for antibiotic coelimycin, which biosynthetic gene cluster is present but not expressed in this strain. Colonies giving a positive biosensor signal appeared at a frequency of ca 10^–5, and all of them were found to produce coelimycin congeners. This result confirmed that our approach can be applied for chemical- and radiation-free mutagenesis in Streptomyces leading to activation of orphan biosynthetic gene clusters and discovery of novel bioactive secondary metabolites.

Expression of an alcohol dehydrogenase gene in a heterotrophic bacterium induces carbon dioxide-dependent high-yield growth under oligotrophic conditions

Sphingobium japonicum strain UT26, whose γ-hexachlorocyclohexane-degrading ability has been studied in detail, is a typical aerobic and heterotrophic bacterium that needs organic carbon sources for its growth, and cannot grow on a minimal salt agar medium prepared without adding any organic carbon sources. Here, we isolated a mutant of UT26 with the ability to grow to visible state on such an oligotrophic medium from a transposon-induced mutant library. This high-yield growth under oligotrophic conditions (HYGO) phenotype was CO₂-dependent and accompanied with CO₂ incorporation. In the HYGO mutant, a transposon was inserted just upstream of the putative Zn-dependent alcohol dehydrogenase (ADH) gene (adhX) so that the adhX gene was constitutively expressed, probably by the transposon-derived promoter. The adhX-deletion mutant (UT26DAX) harbouring a plasmid carrying the adhX gene under the control of a constitutive promoter exhibited the HYGO phenotype. Moreover, the HYGO mutants spontaneously emerged among the UT26-derived hypermutator strain cells, and adhX was highly expressed in these HYGO mutants, while no HYGO mutant appeared among UT26DAX-derived hypermutator strain cells, indicating the necessity of adhX for the HYGO phenotype. His-tagged AdhX that was expressed in Escherichia coli and purified to homogeneity showed ADH activity towards methanol and other alcohols. Mutagenesis analysis of the adhX gene indicated a correlation between the ADH activity and the HYGO phenotype. These results demonstrated that the constitutive expression of an adhX-encoding protein with ADH activity in UT26 leads to the CO₂-dependent HYGO phenotype. Identical or nearly identical adhX orthologues were found in other sphingomonad strains, and most of them were located on plasmids, suggesting that the adhX-mediated HYGO phenotype may be an important adaptation strategy to oligotrophic environments among sphingomonads.

Enrichment, Isolation, and Characterization of High-Affinity N2O-Reducing Bacteria in a Gas-Permeable Membrane Reactor

The recent discovery of nitrous oxide (N₂O)-reducing bacteria suggests a potential biological sink for the potent greenhouse gas N₂O. For an application toward N₂O mitigation, characterization of more isolates will be required. Here, we describe the successful enrichment and isolation of high-affinity N₂O-reducing bacteria using a N₂O-fed reactor (N₂OFR). Two N₂OFRs, where N₂O was continuously and directly supplied as the sole electron acceptor to a biofilm grown on a gas-permeable membrane, were operated with acetate or a mixture of peptone-based organic substrates as an electron donor. In parallel, a NO₃^- -fed reactor (NO₃FR), filled with a nonwoven sheet substratum, was operated using the same inoculum. We hypothesized that supplying N₂O vs NO₃^- would enhance the dominance of distinct N₂O-reducing bacteria. Clade II type nosZ bacteria became rapidly enriched over clade I type nosZ bacteria in the N₂OFRs, whereas the opposite held in the NO₃FR. High-throughput sequencing of 16S rRNA gene amplicons revealed the dominance of Rhodocyclaceae in the N₂OFRs. Strains of the Azospira and Dechloromonas genera, canonical denitrifiers harboring clade II type nosZ, were isolated with high frequency from the N₂OFRs (132 out of 152 isolates). The isolates from the N₂OFR demonstrated higher N₂O uptake rates (V_max: 4.23 × 10^-3-1.80 × 10^-2 pmol/h/cell) and lower N₂O half-saturation coefficients (K_m,N2O: 1.55-2.10 μM) than a clade I type nosZ isolate from the NO₃FR. Furthermore, the clade II type nosZ isolates had higher specific growth rates on N₂O than nitrite as an electron acceptor. Hence, continuously and exclusively supplying N₂O in an N₂OFR allows the enrichment and isolation of high-affinity N₂O-reducing strains, which may be used as N₂O sinks in bioaugmentation efforts.

Nitrogen Cycling in Soybean Rhizosphere: Sources and Sinks of Nitrous Oxide (N2O)

Nitrous oxide (N₂O) is the third most important greenhouse gas after carbon dioxide and methane, and a prominent ozone-depleting substance. Agricultural soils are the primary anthropogenic source of N₂O because of the constant increase in the use of industrial nitrogen (N) fertilizers. The soybean crop is grown on 6% of the world's arable land, and its production is expected to increase rapidly in the future. In this review, we summarize the current knowledge on N-cycle in the rhizosphere of soybean plants, particularly sources and sinks of N₂O. Soybean root nodules are the host of dinitrogen (N₂)-fixing bacteria from the genus Bradyrhizobium. Nodule decomposition is the main source of N₂O in soybean rhizosphere, where soil organisms mediate the nitrogen transformations that produce N₂O. This N₂O is either emitted into the atmosphere or further reduced to N₂ by the bradyrhizobial N₂O reductase (N₂OR), encoded by the nos gene cluster. The dominance of nos ^- indigenous populations of soybean bradyrhizobia results in the emission of N₂O into the atmosphere. Hence, inoculation with nos ⁺ or nos ⁺⁺ (mutants with enhanced N₂OR activity) bradyrhizobia has proved to be promising strategies to reduce N₂O emission in the field. We discussed these strategies, the molecular mechanisms underlying them, and the future perspectives to develop better options for global mitigation of N₂O emission from soils.

Variation in bradyrhizobial NopP effector determines symbiotic incompatibility with Rj2-soybeans via effector-triggered immunity

Genotype-specific incompatibility in legume-rhizobium symbiosis has been suggested to be controlled by effector-triggered immunity underlying pathogenic host-bacteria interactions. However, the rhizobial determinant interacting with the host resistance protein (e.g., Rj2) and the molecular mechanism of symbiotic incompatibility remain unclear. Using natural mutants of Bradyrhizobium diazoefficiens USDA 122, we identified a type III-secretory protein NopP as the determinant of symbiotic incompatibility with Rj2-soybean. The analysis of nopP mutations and variants in a culture collection reveal that three amino acid residues (R60, R67, and H173) in NopP are required for Rj2-mediated incompatibility. Complementation of rj2-soybean by the Rj2 allele confers the incompatibility induced by USDA 122-type NopP. In response to incompatible strains, Rj2-soybean plants activate defense marker gene PR-2 and suppress infection thread number at 2 days after inoculation. These results suggest that Rj2-soybeans monitor the specific variants of NopP and reject bradyrhizobial infection via effector-triggered immunity mediated by Rj2 protein.

Mitigation of soil N2O emission by inoculation with a mixed culture of indigenous Bradyrhizobium diazoefficiens

Agricultural soil is the largest source of nitrous oxide (N2O), a greenhouse gas. Soybean is an important leguminous crop worldwide. Soybean hosts symbiotic nitrogen-fixing soil bacteria (rhizobia) in root nodules. In soybean ecosystems, N2O emissions often increase during decomposition of the root nodules. Our previous study showed that N2O reductase can be used to mitigate N2O emission from soybean fields during nodule decomposition by inoculation with nosZ++ strains [mutants with increased N2O reductase (N2OR) activity] of Bradyrhizobium diazoefficiens. Here, we show that N2O emission can be reduced at the field scale by inoculation with a mixed culture of indigenous nosZ+ strains of B. diazoefficiens USDA110 group isolated from Japanese agricultural fields. Our results also suggested that nodule nitrogen is the main source of N2O production during nodule decomposition. Isolating nosZ+ strains from local soybean fields would be more applicable and feasible for many soybean-producing countries than generating mutants.

Identification of the Hydrogen Uptake Gene Cluster for Chemolithoautotrophic Growth and Symbiosis Hydrogen Uptake in Bradyrhizobium Diazoefficiens

The hydrogen uptake (Hup) system of Bradyrhizobium diazoefficiens recycles the H2 released by nitrogenase in soybean nodule symbiosis, and is responsible for H2-dependent chemolithoautotrophic growth. The strain USDA110 has two hup gene clusters located outside (locus I) and inside (locus II) a symbiosis island. Bacterial growth under H2-dependent chemolithoautotrophic conditions was markedly weaker and H2 production by soybean nodules was markedly stronger for the mutant of hup locus I (ΔhupS1L1) than for the mutant of hup locus II (ΔhupS2L2). These results indicate that locus I is primarily responsible for Hup activity.

The disparity mutagenesis model predicts rescue of living things from catastrophic errors

In animals including humans, mutation rates per generation exceed a perceived threshold, and excess mutations increase genetic load. Despite this, animals have survived without extinction. This is a perplexing problem for animal and human genetics, arising at the end of the last century, and to date still does not have a fully satisfactory explanation. Shortly after we proposed the disparity theory of evolution in 1992, the disparity mutagenesis model was proposed, which forms the basis for an explanation for an acceleration of evolution and species survival. This model predicts a significant increase of the mutation threshold values if the fidelity difference in replication between the lagging and leading strands is high enough. When applied to biological evolution, the model predicts that living things, including humans, might overcome the lethal effect of accumulated deleterious mutations and be able to survive. Artificially derived mutator strains of microorganisms, in which an enhanced lagging-strand-biased mutagenesis was introduced, showed unexpectedly high adaptability to severe environments. The implications of the striking behaviors shown by these disparity mutators will be discussed in relation to how living things with high mutation rates can avoid the self-defeating risk of excess mutations.

Relationship between soil type and N₂O reductase genotype (nosZ) of indigenous soybean bradyrhizobia: nosZ-minus populations are dominant in Andosols

Bradyrhizobium japonicum strains that have the nosZ gene, which encodes N2O reductase, are able to mitigate N2O emissions from soils (15). To examine the distribution of nosZ genotypes among Japanese indigenous soybean bradyrhizobia, we isolated bradyrhizobia from the root nodules of soybean plants inoculated with 32 different soils and analyzed their nosZ and nodC genotypes. The 1556 resultant isolates were classified into the nosZ+/nodC+ genotype (855 isolates) and nosZ-/nodC+ genotype (701 isolates). The 11 soil samples in which nosZ- isolates significantly dominated (P < 0.05; the χ(2) test) were all Andosols (a volcanic ash soil prevalent in agricultural fields in Japan), whereas the 17 soil samples in which nosZ+ isolates significantly dominated were mainly alluvial soils (non-volcanic ash soils). This result was supported by a principal component analysis of environmental factors: the dominance of the nosZ- genotype was positively correlated with total N, total C, and the phosphate absorption coefficient in the soils, which are soil properties typical of Andosols. Internal transcribed spacer sequencing of representative isolates showed that the nosZ+ and nosZ- isolates of B. japonicum fell mainly into the USDA110 (BJ1) and USDA6 (BJ2) groups, respectively. These results demonstrated that the group lacking nosZ was dominant in Andosols, which can be a target soil type for an N2O mitigation strategy in soybean fields. We herein discussed how the nosZ genotypes of soybean bradyrhizobia depended on soil types in terms of N2O respiration selection and genomic determinants for soil adaptation.

The nitrate-sensing NasST system regulates nitrous oxide reductase and periplasmic nitrate reductase in Bradyrhizobium japonicum

The soybean endosymbiont Bradyrhizobium japonicum is able to scavenge the greenhouse gas N2O through the N2O reductase (Nos). In previous research, N2O emission from soybean rhizosphere was mitigated by B. japonicum Nos(++) strains (mutants with increased Nos activity). Here, we report the mechanism underlying the Nos(++) phenotype. Comparative analysis of Nos(++) mutant genomes showed that mutation of bll4572 resulted in Nos(++) phenotype. bll4572 encodes NasS, the nitrate (NO3(-))-sensor of the two-component NasST regulatory system. Transcriptional analyses of nosZ (encoding Nos) and other genes from the denitrification process in nasS and nasST mutants showed that, in the absence of NO3(-) , nasS mutation induces nosZ and nap (periplasmic nitrate reductase) via nasT. NO3(-) addition dissociated the NasS-NasT complex in vitro, suggesting the release of the activator NasT. Disruption of nasT led to a marked decrease in nosZ and nap transcription in cells incubated in the presence of NO3(-). Thus, although NasST is known to regulate the NO3(-)-mediated response of NO3(-) assimilation genes in bacteria, our results show that NasST regulates the NO3(-) -mediated response of nosZ and napE genes, from the dissimilatory denitrification pathway, in B. japonicum.

Bradyrhizobium ottawaense sp. nov., a symbiotic nitrogen fixing bacterium from root nodules of soybeans in Canada

Sixteen strains of symbiotic bacteria from root nodules of Glycine max grown in Ottawa, Canada, were previously characterized and placed in a novel group within the genus Bradyrhizobium. To verify their taxonomic status, these strains were further characterized using a polyphasic approach. All strains possessed identical 16S rRNA gene sequences that were 99.79 % similar to the closest relative, Bradyrhizobium liaoningense LMG 18230(T). Phylogenetic analysis of concatenated atpD, glnII, recA, gyrB, rpoB and dnaK genes divided the 16 strains into three multilocus sequence types that were placed in a highly supported lineage distinct from named species of the genus Bradyrhizobium consistent with results of DNA-DNA hybridization. Based on analysis of symbiosis gene sequences (nodC and nifH), all novel strains were placed in a phylogenetic group with five species of the genus Bradyrhizobium that nodulate soybeans. The combination of phenotypic characteristics from several tests including carbon and nitrogen source utilization and antibiotic resistance could be used to differentiate representative strains from recognized species of the genus Bradyrhizobium. Novel strain OO99(T) elicits effective nodules on Glycine max, Glycine soja and Macroptilium atropurpureum, partially effective nodules on Desmodium canadense and Vigna unguiculata, and ineffective nodules on Amphicarpaea bracteata and Phaseolus vulgaris. Based on the data presented, we conclude that our strains represent a novel species for which the name Bradyrhizobium ottawaense sp. nov. is proposed, with OO99(T) ( = LMG 26739(T) = HAMBI 3284(T)) as the type strain. The DNA G+C content is 62.6 mol%.

Linked expressions of nap and nos genes in a Bradyrhizobium japonicum mutant with increased N(2)O reductase activity

To understand the mechanisms underlying the increased N2O reductase activity in the Bradyrhizobium japonicum 5M09 mutant from enrichment culture under N2O respiration, we analyzed the expression of genes encoding denitrification reductases and regulators. Our results suggest a common regulation of nap (encoding periplasmic nitrate reductase) and nos (encoding N2O reductase).

Implications of fidelity difference between the leading and the lagging strand of DNA for the acceleration of evolution

Without exceptions, genomic DNA of living organisms is replicated using the leading and the lagging strand. In a conventional idea of mutagenesis accompanying DNA replication, mutations are thought to be introduced stochastically and evenly into the two daughter DNAs. Here, however, we hypothesized that the fidelity of the lagging strand is lower than that of the leading strand. Our simulations with a simplified model DNA clearly indicated that, even if mutation rates exceeded the so-called threshold values, an original genotype was guaranteed in the pedigree and, at the same time, the enlargement of diversity was attained with repeated generations. According to our lagging-strand-biased-mutagenesis model, mutator microorganisms were established in which mutations biased to the lagging strand were introduced by deleting the proofreading activity of DNA polymerase. These mutators (“disparity mutators”) grew normally and had a quick and extraordinarily high adaptability against very severe circumstances. From the viewpoint of the fidelity difference between the leading and the lagging strand, the basic conditions for the acceleration of evolution are examined. The plausible molecular mechanism for the faster molecular clocks observed in birds and mammals is discussed, with special reference to the accelerated evolution in the past. Possible applications in different fields are also discussed.

N(2)O emission from degraded soybean nodules depends on denitrification by Bradyrhizobium japonicum and other microbes in the rhizosphere

A model system developed to produce N(2)O emissions from degrading soybean nodules in the laboratory was used to clarify the mechanism of N(2)O emission from soybean fields. Soybean plants inoculated with nosZ-defective strains of Bradyrhizobium japonicum USDA110 (ΔnosZ, lacking N(2)O reductase) were grown in aseptic jars. After 30 days, shoot decapitation (D, to promote nodule degradation), soil addition (S, to supply soil microbes), or both (DS) were applied. N(2)O was emitted only with DS treatment. Thus, both soil microbes and nodule degradation are required for the emission of N(2)O from the soybean rhizosphere. The N(2)O flux peaked 15 days after DS treatment. Nitrate addition markedly enhanced N(2)O emission. A (15)N tracer experiment indicated that N(2)O was derived from N fixed in the nodules. To evaluate the contribution of bradyrhizobia, N(2)O emission was compared between a nirK mutant (ΔnirKΔnosZ, lacking nitrite reductase) and ΔnosZ. The N(2)O flux from the ΔnirKΔnosZ rhizosphere was significantly lower than that from ΔnosZ, but was still 40% to 60% of that of ΔnosZ, suggesting that N(2)O emission is due to both B. japonicum and other soil microorganisms. Only nosZ-competent B. japonicum (nosZ+ strain) could take up N(2)O. Therefore, during nodule degradation, both B. japonicum and other soil microorganisms release N(2)O from nodule N via their denitrification processes (N(2)O source), whereas nosZ-competent B. japonicum exclusively takes up N(2)O (N(2)O sink). Net N(2)O flux from soybean rhizosphere is likely determined by the balance of N(2)O source and sink.

Whole-Genome Profiling of a Novel Mutagenesis Technique Using Proofreading-Deficient DNA Polymerase δ

A novel mutagenesis technique using error-prone DNA polymerase δ (polδ), the disparity mutagenesis model of evolution, has been successfully employed to generate novel microorganism strains with desired traits. However, little else is known about the spectra of mutagenic effects caused by disparity mutagenesis. We evaluated and compared the performance of the polδMKII mutator, which expresses the proofreading-deficient and low-fidelity polδ, in Saccharomyces cerevisiae haploid strain with that of the commonly used chemical mutagen ethyl methanesulfonate (EMS). This mutator strain possesses exogenous mutant polδ supplied from a plasmid, tthereby leaving the genomic one intact. We measured the mutation rate achieved by each mutagen and performed high-throughput next generation sequencing to analyze the genome-wide mutation spectra produced by the 2 mutagenesis methods. The mutation frequency of the mutator was approximately 7 times higher than that of EMS. Our analysis confirmed the strong G/C to A/T transition bias of EMS, whereas we found that the mutator mainly produces transversions, giving rise to more diverse amino acid substitution patterns. Our present study demonstrated that the polδMKII mutator is a useful and efficient method for rapid strain improvement based on in vivo mutagenesis.

Bacterial hypermutation: clinical implications

Heritable hypermutation in bacteria is mainly due to alterations in the methyl-directed mismatch repair (MMR) system. MMR-deficient strains have been described from several bacterial species, and all of the strains exhibit increased mutation frequency and recombination, which are important mechanisms for acquired drug resistance in bacteria. Antibiotics select for drug-resistant strains and refine resistance determinants on plasmids, thus stimulating DNA recombination via the MMR system. Antibiotics can also act as indirect promoters of antibiotic resistance by inducing the SOS system and certain error-prone DNA polymerases. These alterations have clinical consequences in that efficacious treatment of bacterial infections requires high doses of antibiotics and/or a combination of different classes of antimicrobial agents. There are currently few new drugs with low endogenous resistance potential, and the development of such drugs merits further research.